A GP 100 in .357 is a solid choice - and realistically offers just as much utility as a semi-auto. In scenarios that my time in the bush has come up with, 6 rounds versus 10 rounds on board is not all that much of a difference - and magazine versus speedloader is even less relevant.
In terms of dirt/snagging/magazines being released - I've had a bit of pertinent experience in that area. I've ended up crawling through some truly ugly brush in Texas while hunting feral hogs, and did so with a Glock 20 on my belt. When in a holster which is selected with that sort of duty in mind, there is no way that the magazine could be released as long as the pistol is in the holster, because the magazine release button is only accessible from the left side of the pistol (assuming a right handed shooter) - and that area of the right side of the pistol is in contact with the material of the holster.
If you've already purchased the GP100, then I see absolutely zero issue to change course. It is an eminently serviceable revolver, and for your application the only relevant advantage that a Glock 20 would offer is being lighter. If you have not yet purchased it, you may want to take a close look at a variety of holster options - and maybe even talk to one of the Canadian outfits that makes Kydex holsters to order. Despite enjoying quality leather, I like kydex for holsters for a few reasons - one-handed re-holstering being at the top of the list. It's also impervious to water, can be cleaned with a garden hose and brush no matter how filthy it has gotten, and with good design you can adjust the tension in a variety of ways.
Speaking as someone who has spent a lot more time with a pistol on my belt than most Canadians, your holster and belt choice probably matters more in your level of comfort and "pain in the @rse factor" with carrying a handgun than the difference between a G20 and a GP100. I found that a G20 with the right holster and belt was more comfortable and less obtrusive to carry than a single stack 9mm in the wrong holster.